This application is based on and claims priority from Japanese Application No. HEI 2-335937 filed Nov. 30, 1990, the disclosure of which is incorporated by reference herein.
The present invention relates to an afocal optical system for use with binoculars and monoculars, particularly to a zooming optical system capable of changing the magnification.
Conventional zooming optical systems have their zooming ability distributed between the eyepiece optical unit and the objective optical unit, and the distribution ratio is usually higher for the eyepiece unit. This is because zooming by the eyepiece unit provides ease in correcting aberrations.
However, if the eyepiece optical unit has a higher zooming ability than the objective unit, the apparent "field of view" will vary. The user of a binocular or similar optical instrument senses subjectively the magnification of an image as the relationship between the frame defining the field and the size of the object. Hence, he feels a change of image magnification if the object magnification changes but the apparent field remains the same. However, a change in image magnification is difficult to perceive if both the object magnification and the field size vary as in the conventional zooming optical system.
Another problem with a conventional afocal zooming optical system is that its overall length is apt to increase because it is necessary to ensure not only a long distance (eye relief) between the face of the eye lens that is the closest to the eye of the viewer and an optimal position for looking through the viewfinder, but also a sufficient space for zooming. In order to realize a compact system, the powers of the individual lens groups must be increased, but then it becomes difficult to correct aberrations.